The R-T-B based sintered magnet (R represents rare earth element(s), T represents Fe or Fe with part of it replaced by Co, and B represents boron) with the tetragonal compound R2T14B being its main phase is known to have excellent magnetic properties and thus is a representative permanent magnet with high performances since it was invented in 1982 (Patent Document 1).
The R-T-B based sintered magnet with the rare earth element(s) R being composed of Nd, Pr, Dy, Tb and/or Ho has a large magnetic anisotropy field Ha and is preferably used as a permanent magnet material. Especially the Nd—Fe—B based permanent magnet with Nd being the rare earth element R is widely used in consumer, industries, transportation equipments and the like because it has a good balance among the saturation magnetization Is, the Curie temperature Tc and magnetic anisotropy field Ha. Also, it is better than other R-T-B based sintered magnets in which other rare earth element(s) is/are used from the view point of the resource amount and corrosion resistance.
The improvement of magnetic properties is required in the conventional R-T-B based magnet. Particularly, a lot of efforts have been taken to improve the residual magnetic flux density Br and the coercivity HcJ. As one of the employed methods, a method is proposed that element(s) having high magnetic anisotropy such as Dy or Tb is/are added to increase the coercivity.
However, from the viewpoints of resource saving and cost reduction, the amount of the added heavy rare earth element(s) is required to be kept to a minimum. As the method for adding the heavy rare earth element(s), for example, a technique involving grain boundary diffusion has been disclosed (Patent Document 2).
As another method for adding the heavy rare earth element(s), a technique has been disclosed in which the RH-T phase (RH represents the heavy rare earth element) is mixed with the RL-T-B phase (RL represents the light rare earth element) or alternatively the RH-T-B phase is mixed with the RL-T-B phase to manufacture the sintered compact (Patent Document 3).